Abstract
Mammalian birth is accompanied by a period of obligatory asphyxia, which consists of hypoxia (drop in blood O2 levels) and hypercapnia (elevation of blood CO2 levels). Prolonged, complicated birth can extend the asphyxic period, leading to a pathophysiological situation, and in humans, to the diagnosis of clinical birth asphyxia, the main cause of hypoxic-ischemic encephalopathy (HIE). The neuroendocrine component of birth asphyxia, in particular the increase in circulating levels of arginine vasopressin (AVP), has been extensively studied in humans. Here we show for the first time that normal rat birth is also accompanied by an AVP surge, and that the fetal AVP surge is further enhanced in a model of birth asphyxia, based on exposing 6-day old rat pups to a gas mixture containing 4% O2 and 20% CO2 for 45 min. Instead of AVP, which is highly unstable with a short plasma half-life, we measured the levels of copeptin, the C-terminal part of prepro-AVP that is biochemically much more stable. In our animal model, the bulk of AVP/copeptin release occurred at the beginning of asphyxia (mean 7.8 nM after 15 min of asphyxia), but some release was still ongoing even 90 min after the end of the 45 min experimental asphyxia (mean 1.2 nM). Notably, the highest copeptin levels were measured after hypoxia alone (mean 14.1 nM at 45 min), whereas copeptin levels were low during hypercapnia alone (mean 2.7 nM at 45 min), indicating that the hypoxia component of asphyxia is responsible for the increase in AVP/copeptin release. Alternating the O2 level between 5 and 9% (CO2 at 20%) with 5 min intervals to mimic intermittent asphyxia during prolonged labor resulted in a slower but quantitatively similar rise in copeptin (peak of 8.3 nM at 30 min). Finally, we demonstrate that our rat model satisfies the standard acid-base criteria for birth asphyxia diagnosis, namely a drop in blood pH below 7.0 and the formation of a negative base excess exceeding −11.2 mmol/l. The mechanistic insights from our work validate the use of the present rodent model in preclinical work on birth asphyxia.
Highlights
In all mammalian species, the shift during parturition from maternal-fetal umbilical respiratory gas exchange to the activation of fetal lungs is associated with a transient period of asphyxia in the neonate
We demonstrate that this model satisfies the standard diagnostic criteria (Azzopardi et al, 2009; Schlapbach et al, 2011; Summanen et al, 2017) of human birth asphyxia based on blood pH and negative base excess (BE)
Several studies have shown that arginine vasopressin (AVP)/copeptin levels are increased after normal vaginal birth in human neonates (Chard et al, 1971; Wellmann et al, 2010), whereas neonates born by elective cesarean section generally have very low serum copeptin levels (Wellmann et al, 2010)
Summary
The shift during parturition from maternal-fetal umbilical respiratory gas exchange to the activation of fetal lungs is associated with a transient period of asphyxia in the neonate. Asphyxia implies a fall in blood O2 (hypoxia) that is associated with an elevation of CO2 (hypercapnia) This period of obligatory, non-pathophysiological asphyxia is Copeptin in Rat Birth Asphyxia Model an essential part of the “stress of birth” (Van Woudenberg et al, 2012; Evers and Wellman, 2016), which is beneficial in that it activates the hypothalamic-pituitary axis (the HPA axis) as well as the sympathetic nervous system. A prolonged period of birth asphyxia is harmful, causing dysfunction and damage with lifelong consequences in organ systems with a high and predominantly aerobic energy metabolism, such as the brain (Painter, 1995; Fattuoni et al, 2015; Ahearne et al, 2016). In order to understand the physiological mechanisms causing HIE, and to develop therapies for the treatment of HIE, a wide variety of animal models ranging from standard laboratory rodents to large mammals such as piglets and sheep have been used to explore the mechanisms, as well as short- and long-term consequences, of birth asphyxia (Raff et al, 1991; Painter, 1995; Vannucci and Vannucci, 1997; Johnston et al, 2005; Fattuoni et al, 2015; Mallard and Vexler, 2015)
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